The invention relates to the field of petroleum service and supply industries, and more particularly to completing wells for producing hydrocarbons, geothermal wells, or the like.
While drilling is taking place, the integrity of a well is controlled by a drilling mud of density that needs to be adjusted so that the hydraulic pressure of the mud column opposes the leaks from the formations while simultaneously avoiding damaging the underground formations by fracturing them. When the drilled depth exceeds a certain value, the pressure difference due to the difference in depth is such that it is no longer possible to formulate a mud capable of performing its function over the entire length of the well, so to prevent collapse of the wall, it is necessary to line the hole with metal casing. For this purpose, a certain number of casing tubes are placed end to end and lowered down the well, and are fixed to the wall of the well by cementing. Thereafter, drilling can continue down to the next critical depth.
Each newly-drilled length must be lined with casing of outside diameter that is small enough to pass through the casing that is already in place at shallower depths. As a result the casing has a staircase structure with a hole that is large at the top of the well and much narrower at the bottom of the well. Such a configuration is far from optimal: a large hole at the surface means that drilling time must be wasted in a non-productive zone, whereas a narrow hole in the useful zones does not favor production by good draining of the formation.
Worse still, it often happens that the hole passes through unexpected critical zones even before boring has reached a critical depth. Such critical zones may, for example, be veins of very friable rock or "pockets" of gas which, even though they are usually very localized, constitute major sources of danger both for the well and for the work force on the surface. Under such circumstances, the only solution is often to cement these zones by putting casing into place immediately, thereby further reducing the size of the hole, which can lead to a well being abandoned if further difficult zones are encountered as drilling continues.
It will also be understood that it is very difficult to repair damaged casing by installing new casing, without further significantly reducing the size of the hole and thus running the risk of preventing penetration of certain tools or items of equipment that may be needed in the production zones, for example.
Over the last few years, the industry has developed new techniques of completing wells or of completing them temporarily, so as to minimize the number of "steps" and to increase the downhole diameter of the well.
Proposals have thus been made to use a composite material comprising an expandable cloth made of glass fibers impregnated with non-polymerized epoxy resin and having a rubber membrane covering its outside face which is directed towards the wall of the hole. By using an appropriate laying tool, the membrane is applied against the wall of the hole or a damaged portion of casing, and the resin is caused to polymerize by being heated. The main difficulty of that technique is that it requires electrical power of the order of 1000 watts per linear meter, thus limiting its application to treating zones that are relatively short. In addition, such a casing of synthetic material cannot constitute a final replacement for metal casing since that needs to be capable, in particular, of withstanding treatments based on strong acids or other materials that are particularly corrosive.
US patent U.S. Pat. No. 5,348,095 proposes making casing out of a continuous tube of ductile material capable of withstanding large amounts of plastic deformation. The tube is enlarged by a conical tool, and it can optionally be cemented. However, expansion of the tube is accompanied by a reduction in the total length of the tube, and this can give rise to interface problems at the ends. Furthermore, the pressure required for expanding the tube is very high.
Patent U.S. Pat. No. 5,366,012 proposes a perforated liner provided with overlapping longitudinal slots. A mandrel having a large diameter that is greater than the inside diameter of the perforated liner is used to expand the wall of the liner and the orifices become larger. Fiber-reinforced cement can then be cast on either side of the liner, and once the cement has set, the inside of the liner is bored again, thereby leaving a casing of fibro-cement that is reinforced by metal reinforcement. The need for further boring after cementing constitutes a major drawback of that technique. In addition, in the above-mentioned case, the pressures required for expanding the tube are quite high and the final length of the liner is reduced. Finally, the slots must be pierced in compliance with very precise specifications, which leads to a manufacturing cost that is high.
An object of the present invention is a novel type of expandable casing that does not present the above-mentioned drawbacks of the art.